Lap siding is installed on a building by nailing the siding to the exterior surface of the building wall. Particularly with vinyl and aluminum siding, channels are used to cover the ends of the siding. Thus, when siding abuts a door frame, window frame, building corner, or edge of a vent, a channel is provided to cover the cut end of the siding. The channel can be a separate article or can mold into an article.
These channels have a C-shaped cross-section formed from three elongated narrow strips, a nailing strip, an abutment strip which extends up from the nailing strip and then a covering strip which extends parallel to the nailing strip. The siding is nailed to the exterior of the building with the cut ends extending into these channels. The purpose of this is to provide some play in the sizing of the pieces of siding. This reduces the required accuracy. If the siding is cut half inch too short, the channel will cover this and the final product has a very professional, finished appearance.
Unfortunately, water can flow from upper horizontal portions of a channel into the vertical channel. Once in the vertical channel, the water can run down the channel and back behind the siding. This, of course, causes moisture damage to the wall of the building and can even damage the interior of the building. Further, if enough water accumulates behind the siding and freezes, it will cause the siding to separate from the building.